In general, digital age compression comprises encoding an entire image, either as a whole or in a block-wise manner, to produce a bit-stream of data representing the image. Often the size, measured in bits, of the bit-stream representing the image is reduced, by the compression process, when compared with the number of bits required to represent the digital image in an uncompressed format.
An advantage of compressing a digital image is that a compressed digital image data physically occupies less storage space on a storage device (eg. RAM, Disk or Magnetic Tape storage). Another advantage lies in a transmission of the digital images; a reduced bit-stream, typically as a result of compression, requires less transmission time than does the transmission of the digital image in an uncompressed format. At least one drawback of storing or transmitting a digital image in a compressed format is that the compressed image requires processing or decompressing before it can be displayed on a display device. Consequently, a compressed digit image cannot be displayed without investing costly time to decompress the compressed image. When browsing through a multitude of images, which are stored in a storage device or transmitted to a user in compressed format, having to decompress each image and display the image to find a select one of the images can pose a difficulty. In particular, the processing time required to decompress each image can render the browsing slow and cumbersome.
A number of hierarchical techniques for image coding are known which involve coding a lower resolution version of an image within a bit stream of a higher resolution version of the image. However, to display the lower resolution version of an image the compressed image or coded bit stream must be decoded. Typically the lower resolution version of an image can not be gleaned off the coded bit stream in a form substantially ready for display without decoding the entire, or part of the image. Further, with many of the known techniques, a low resolution version of an image in an uncoded format (ie. a format substantially ready for display) cannot be embedded in a bit stream of a coded higher resolution version of the image without substantially affecting the compression efficiency, computational efficiency or decoding complexity.
In addition, for known techniques where a first (low resolution) image is stored together with a second (high resolution) image, the first image typically being a small size likeness of the second image, the first image generally duplicates information already inherent in the second image. For example, a digital image can be duplicated and the duplicate image sub-sampled to provide a small likeness (lower resolution) of the digital image. The digital image is compressed and the small duplicate likeness can be stored in an uncompressed format together with the compressed digital image. The small duplicate likeness of the digital image makes available a representation of the digital image readily displayable on a display device without a requirement to decompress the entire digital image. However in current techniques the small duplicate likeness of the digital image is typically stored as an independent stream of data and not as an uncompressed portion of a digital image. Thus with the known techniques, at least, some disadvantages lie in a requirement to store duplicate information inherent to a digital image, which reduces the compression efficiency.